Frequency measuring system



June 1943- H. o. PETERSON ETAL 2,321,315

FREQUENCY MEASU'RING SYSTEM Patented June 8, 1943 FREQUENCY MEASURINGSYSTEM Harold 0. Peterson and John B. Atwood, Riverhead, N. Y.,assignors to Radio Corporation of America, a corporation of DelawareApplication May 9, 1941, Serial No. 392,624

filaims.

The present invention relates to frequency measurement and, moreparticularly, to a means for measuring radio frequencies. i

An object of the present invention is the measurement of radiofrequencies.

A further object of the present invention is the accurate measurement ofradio frequencies to the nearest whole number of cycles.

Another object of the present invention is the provision of means formeasuring the periodicity of radio frequency waves with extremeaccuracy.

Still another object of the present invention is the provision ofequipment for measuring the periodicity of high frequency waves wherebythe measuring equipment may be conveniently and quickly checked as tostability of operation.

The present invention contemplates measurement of the periodicity of aradio frequency wave by receiving the wave, heterodyning therewith thewave generated by the oscillator of a superheteroclyne receiver which iscoupled to the output of a harmonic generator operating from a 10kilocycle standard frequency source thus determining the frequency ofthe wave to the nearest 10 kilocycles. The difference between thefrequency of the oscillator when the receiver is tuned to the nearestharmonic and the frequency of the received wave is then compared withthe output of an interpolation oscillator having separate independentcalibrated controls for varying the frequency over ranges of 2kilocycles and kilocycles. The controls are varied to obtain zero beatand the calibration of the controls read to determine the periodicity ofthe received radio frequency wave to the nearest whole cycle.

Means are also provided for comparing the waves generated by thedifferent oscillators with that generated by the standard frequencysource so that the effects of oscillator drift may be overcome.

The present invention will be more clearly understood by reference tothe following detailed description, which is accompanied by drawings inwhich Figure 1 illustrates a block diagram of a system for practicingthe present invention;

, Figure 1a illustrates in block diagram form and in greater detail aportion of Figure 1, Figure 2 illustrates in more detail theinterpolation oscillator of Figure 1, while Figures 3 and 4 illustratemodifications of the invention.

Referring, now, to the block diagram in Figure 1 at the upper right handcorner there is a radio frequency amplifier l0 covering the frequencyrange of 3 to 24 megacycles. An input of the amplifier is permanentlyconnected to a harmonic generator l2 operating from the standard crystaloscillator l4 which supplies standard frequencies of ten and one hundredkilocycles. The input l5 of the radio frequency amplifier l0 may beconnected to an antenna 16 of appropriate constants for the signal, thefrequency of which is to be measured. The output of radio frequencyamplifier l0 appears in channel I8 as. a wave having a frequency of theorder of 1500 kilocycles, and is applied to the. interpolationoscillator 20. The input of the interpolation oscillator 20 may beselectively switched by means of switch 2| to two channels, one ofwhich, I8, is the output of the 3 to 24 megacycle radio frequencyamplifier Ill; theother of which, 22, is

the output of a kilocycle to 3 megacycle radio frequency amplifier 4.This last mentioned amplifier, it will be seen, is arranged so that itmay be connected directly to an antenna or through phase shifter 21 totwo. antennas by means of switch 28. Phase shifter 21 is often used.when measuring the frequency of a broadcast transmitter (550 kilocyclesto 1500 kilocycles). Its purpose is to eliminate interference from asecond transmitter on the same channel. For a further description of itsoperation reference may be had to a copending H. 0. Peterson application#236,027, filed October 20, 1938. It is used only in the broadcast bandand hence is not shown in connection with radio frequency amplifier Ill.The output of amplifier 4 appears as a wave of the order of 3.5megacycles. In order that this output may be applied to theinterpolation oscillator 20 it is converted to a wave of a frequency ofthe order of 1500 kilocycles by frequency converter 30. This is done bybeating the 3.5 megacycle wave from amplifier 4 against a 2' megacyclewave generated in generator 32 from the 100 kilocycle standard sourcel4. The difference frequency of the order of 1500 kilocycles is fed bymeans of channel 22 to the switch 2| on the interpolation oscillatorinput. It will thus beseen that a wave of any frequency from 100kilocycles to 24 megacycles may be picked up by an appropriate antennaand applied to the input of the interpolation oscillator.

The interpolation oscillator 20 operates at a frequency variable 12kilocycles above and below a nominal base frequency of 1050 kilocycles.This frequency variation iscalibrated and the calibration .is used tomeasure the frequency of the incoming signal by a method which will beexplained hereafter in detail. The input signal to be measured, whichnow is at a frequency of the order of 1500 kilocycles appearing at theinput of the interpolation oscillator unit, also is applied throughchannel 33 to a frequency converter 34. The purpose of this will also bedescribed in detail hereafter.

The wave generated by the interpolation oscillator having a frequency of1050112 kilocycles is heterodyned with the 1500 kilocycle signalappearing at its input to produce a wave having an intermediatefrequency of 450 kilocycles, which is applied through channel to a firstintermediate frequency amplifier 35.

Wherever, in this application, frequencies of 1500 kilocycles,kilocycles, etc., are referred to as appearing in the variousintermediate frequency channels it should be distinctly understood thatthis is only a nominal value and the actual frequency may vary from thiswithin the passband of the chamiel involved.

As shown in more detail in Figure la, the first intermediate frequencyamplifier 36 has included Q therein, in addition to the amplifierchannel 35, a frequency converter 3! and an oscillator 33. The-400kilocycle. wave generated by oscillator 38 is combined in converter.S'Iwith the nominal 450 kilocycle input to amplifier 35 to produceanother wave having a base frequency of 50 kilocycles. To the output ofconverter 31 is connected a channel 39 having therein a variableband-pass filter 50 which may be varied to provide three differentdegrees of selectivity. The amplifier 35 also contains a monitoringsystem forlistening to the incoming signal with a pair oiKhead phones4!, the signal being brought down to audio frequency for this purpose bybeating it against a 50 kilocycle Wave from oscillator 42.

frequency amplifier 36, these being the audio monitoring channel 43,the. 50 kilocycle signal channel 4! and the 50 kilocycle oscillatoroutput channel 45. oscilloscopes. A7, at and 69, as shown inv bothFigures 1, 1a.

Returning, now, to. Figure. 1, the frequency converter 34, the input ofwhich is in parallel with the interpolation oscillator 20 is intended tobe used as a monitor .on any drift in the oscillators throughout the.system during the period of a frequency measurement. The 1500 kilocycleinput to the converter 34 appearing on channel 33 is changed to a 450kilocycle intermediate frequency by. beating itin the converter againsta 1050 kilocycle wave produced in generator from the 10 kilocyclestandard frequency source I4. The 450 kilocycle output from converter 34is applied through connection 55 to the input of a second wherein it isconverted to 50kilocycles by beating against the 400 kilocycle waveproduced by oscillator 38in the first mentioned intermediate frequencyamplifier. The second intermediate frequency amplifier 58 is asimplified version of the first as it hasonly one band width andcontains no oscillators or monitoring equipment. The 50 kilocycle outputfrom the second intermediate amplifier 53 is applied by means of channelto the other pair of plates of the oscilloscope 47 than those fed by the50 kilocycle oscillator 42 in the first intermediate frequency amplifier36.

The interpolation oscillator will now be d escribed in more detail byreference to Figure 2 of the drawings. The unit actually consists of twoseparate oscillators, one oscillator, identified by reference numeral65, operating within a band :11 kilocycles from 250 kilocycles asamidband frequency and the other, identified by reference character'lil,operating at800 kilocycles and var- Three output-channels are-shown fromthis portion of theintermediate These three outputs are, fed to threeintermediate frequencyamplifier 58 iable 11 kilocycles above and belowthat value. The output of the two oscillators and 10 is applied toseparate grids, for example, grids 8| and 83 of a mixer tube to producethe desired nominal frequency of 1050 kilocycles at the anode 86 of themixer tube 80. The output wave at a frequency of 1050 kilocycles frommixer tube 80 is filtered in the tuned circuits 8'! and 89, forming theinput and output circuits of an amplifier 88. The filter circuits arearranged to pass a frequency band of :12 kilocycles with a midbandfrequency of 1050 kilocycles. The resultant output Wave is combined inanother mixer tube 90 with the incoming 1500 kilocycle signal fromchannels [0 or 22 after being amplified as much as desired or considerednecessary, as indicated by amplifier 92. A band-pass filter arrangement93 is included between amplifier 92 and the mixer 90 to pass on thefrequencies of 1500 kilocycles :12 kilocycles. The output of mixer tube90 then appears in output channel 35 within the desired kilocycleintermediate frequency wave band.

The 250 kilocycle oscillator 65 is variable in frequency -l.l kilocyclesand the 800 kilocycle oscillator i0 is variable in frequency :11kilocycles from their nominally given midband frequencies. Thecombination of these two frequencies produces a resultant output of 1050kilocyclesilZl lrilocycles. The tuning controls 61 and II on the twooscillators 65 and 10 are calibrated in frequency, the control for the250 kilocycle oscillator every cycle and the control for the 800kilocycle oscillator ever 50 cycles. be measured to the nearest cycle,the 50 cycle calibration points on this oscillator are disregarded andits frequency changed by integral multiples of 1000'cycles, this beingset precisely by comparison of the audio output of the receive with the1000 cycle standard by means of oscilloscope 49. The cycles are thenobtained by what amounts to interpolation between the 1000 cycle pointsby means of the 250 kilocycle oscillator which is calibrated to cycles.The reason for using two separate oscillators for the purpose willbecome apparent when the method of measuring is discussed hereafter.

Let us assume, for the purpose of illustration, that a frequency of3743.651 kilocycles is to be measured. The two tuning controls on theinterpolation oscillator, that is, controls SI and 'H, are set to zeroat which point the output frequency is 1050 kilocycles; Then using the 3to 2 1 megacycle radio frequency amplifier iii, the nearest l0 kilocycleharmonic from the harmonic generator i2 is tuned in. This harmonic is3740 kilocycles. In order to set this accurately to zero beat thepattern on oscilloscope G8 is observed. This oscilloscope compares thefrequency of the 50 kilocycle signal produced in the first intermediatefrequency amplifier 35 with the output of the 50 kllocycle oscillator 42used to produce zero beat forthe audio monitor. Zero beat is indicatedby a stationary single loop pattern on the screen produces a wave of afrequency of 46.349 kilo If frequencies are to.

cycles. Finally, beating this with the 50kilocycle oscillator 42produces a 3.651 kilocycle tonein the audio output.

Now, if the frequency of the 1050 kilocyclein terpolation oscillator 20is varied to 1046.349 kilocycles it will beat with the 1496.349kilocycle signal coming from the radio frequency tuner It] to produce a450 kilocycle intermediate frequency which will produce zero beat in thephones M. The zero beat will also be observed as a single loop patternon oscilloscope 48.

The amount the interpolation oscillator frequency has been changed isexactly the same as the difference between thesignal being measured andthis indicates that the frequency must be variable by at leastkilocycles. 11 kilocycles has been selected as a reasonable overlap.

It is also necessary for the accuracy desired that the frequency changeof the interpolation oscillator be readable to the nearest single wholecycle. If this were to be done with a single calibrated control dial itwould require 22,000 divisions, an impractical number. However, if thefrequency change is dividedbetween two controls the first reading 1000cycle points only and the second reading individual cycles betwen the1000 cycle points only 1,000 divisions are required.

It is neecssary to use separate oscillators for the two tuning controlsof the interpolation oscillator since the frequency change produced byeither one must be independent of the other if they are to be calibrateddirectly in terms of frequency change. If the two controls droveparallel variable tuning means in a single oscillator the frequencychange produced by the second condenser would depend upon the setting ofthe first and neither could be directly calibrated in frequency.

Some means must be provided to set the 1000 cycle points on the firstmentioned tunin control accurately and we have arranged this to beaccomplished by means of oscilloscope 49.

It has been heretofore shown how the 3740 kilocycle harmonic was set tozero beat by means of oscilloscope 48 with the controls of interpolationoscillator. set at zero. If the control calibrated in 1000 cycle pointsis now changed by 3000 cycles, a 3000 cycle tone will be heard. If thisis compared with the 1000 cycles from the frequency standard by usingoscilloscope 49, the correct point will be indicated by the appropriatestationary Lissajous pattern produced. Leaving the first tuning controlset and changing the units calibrated control by 651 cycles the signalwill be at zero beat and this is indicated by the pattern onoscilloscope 48. The frequency is then the sum of the standard harmonicfrequency and the readings of the two calibrated tuning controls of theinterpolation oscillator.

Since there is a possibility of any or all of the oscillators in thesystem drifting during the period of the measurement, it is desirable toprovide a continuous monitor on the frequency stability. This we haveaccomplished by providing a parallel set of equipment fed from the inputchannel 33. The signal in this channel 'isap;

plied to a frequency converter 34, whose 1050, kilocycle oscillatorinput is obtained from gen-5 erator 55 controlled by the frequencystandard I4.

As previously described, the 450 kilocycle output of the converter 34 iscombined with the-400 kilocycle wave from the oscillator 38 of the firstintermediate frequency amplifier toproduce a 50 kilocycle wave which isapplied to one set of deflecting plates of the oscilloscope 41 by meansof channel 60. Likewise, the 50 kilocycle signal from the firstintermediate frequency amplifier is applied to the other setofdefiecting plates-of the oscilloscope 41. Q

When the radio frequency amplifier v I0 is tuned to zero beat with a,standard harmonic frequency it produces a fixed pattern on oscilloscope41, as well as on 48. During the measuring of the signal only' theinterpolation oscillator. 2C! is.

varied. Since the stability monitoring system is connected ahead ofinterpolation oscillator 20,

variations of this oscillator. do not affect the monitor circuit. As aresult, the drift of all the oscillators in the receiver, excepttheinterpolation oscillator, will be continuously monitored, the driftbeing indicated by a moving pattern on oscilloscope 41. Any drift may beautomatically corrected by changing the tuning of the amplifier II'Iirrespective of the frequency of the interpolation oscillator 20. Theinterpolation oscillator 20 may be checked for drift by resetting theunits calibrated tuning control to. zero and noting whether or not astationary pattern is produced on oscilloscope 48 when the tuningcontrol H calibrated in 50 cycle units is set at zero, or onoscilloscope 49 if control-II is set at any whole number of kilocyclesother than zero. If desired,;

automatic gain controlmay beusedin the first intermediary frequencyamplifier 36 to 1 help maintain a constant pattern size on oscilloscopes41 and 48 with changes in the incoming signal, level, but automatic gaincontrol is not considered desirable on thesecond intermediate frequencyamplifier 58. as it operates from the constant output of the harmonicgenerator I2.

The operation of .our invention and the indications on the threeoscilloscopes 41, 48 and 49 may be briefly summarized, without.considering the variations infrequencies in the-various channels, in thefollowing manner.

The two control dials on the interpolation oscillator 20 are set to-zeroand a 10 kilocycle.

Now as long as the 10 kilocycle harmonic is present in the radiofrequency amplifier input' and the radio frequency amplifier tuningcontrols are not changed, oscilloscope 4'I continues to show thiscircular pattern if there is no drift in the various oscillators.

If the 1000 cycle control dial of the interpolation oscillator is nowvaried, complex patterns appear on both oscilloscopes Hand-49 untilthis" has changed the received beat frequency by 1000 cycles or amultiple thereof.- At this'point, oscilloscope 48 continues to show acomplex patternandoscilloscope' 49, by an appropriate patterny of theinterpolation oscillator 20 by means'ofj- By means of indicates thenumberof thousand cycles the interpolation oscillator dial has beenmoved. If the cycledial of' the interpolation oscillator is thenchanged,oscilloscopes 48 and 49 show complex patterns.

If a-signal is introduced into radiofrequency amplifier III by means ofantenna I6 and ifitis assumed that the above'describedmanipulation ofthe interpolation oscillator is such as to zerobeat the signal, this isindicated-by a stationary circular pattern on oscilloscope 48.-Oscilloscope 4-I continues to show a circularpattern showing no drift.in the equipment and oscilloscope 49 indicates acomplex pattern.

From the above, it will be seen that the only times oscilloscope 48indicates zero beat are: (A) when both controls of the-interpolationoscillator 20--are at zero andaharmonic is at zero beat, or (B) when theinterpolation oscillatorcontrols are such as to produce a zero beat witha-received signal. a

The accuracy of measurement may be increased 1 and themethod ofmeasurement of frequencies simplifiedat thecost of some increasedcomplexity- -of structure by using a--modified-form ofinterpolationoscillator as hereinafter described-.-

In the invention, as heretofore described, the interpolation oscillatorconsists of two oscillators, one-operatingat 250 kilocyclesiLlkilocycles-and the-other at 800 kilocyclesill kilocycles. On referringto the foregoing description, it may be seen how-it is-desirable tochange the frequency of the-800 kilocycleoscillatorin 1000 cycle stepsand how thisis accomplished by comparing-the audio beat from thereceiver-againstthe standard 1000 cycles by means of oscilloscope 49 inFigure 1.

In order to overcome the possibility of frequency drift of the 800kilocycle oscillator I and the operating inconvenience. ofcarefullysetting the frequency of this-oscillator and the time wasted inso doing an interpolation oscillator as 1 The manner in which the790-810 kilocycle frequencies are obtained and controlled may best beseen by referring to Figure 3;

Starting in the upper right hand corner erator IOI which supplies a 2000cycle wave. This of the diagram, the1000 cycle-standard is filtered bysharp pass filter I00 and fed; toa harmonic gen- This 2000 cycle wave isfiltered atfilter I02. and fed 7 a filter in its output which selectsone-of. the

2 kilocycle harmonics from 30- to 50 kilocycles. For example, amplifier20I is followed byfilter 23I which passes 30 kilocycles. Similar filtersin-the other amplifiers are identified by reference If thesefrequencies. are.

numerals 232-to-24I'. successively combined with a fixed'frequency of760-kilocycles, a series of outputfrequencies varying-inz" kilocyclesteps over the range 790 to 810 I put filters 23I20I are the grids ofthree tubesv in parallel. The reason for three tubes is due tothedesirability of havingthese frequencies available for three separateand distinct frequency measuring receivers A, B and C. Only two sets ofthese tubes are shown on the diagram to avoid confusion.

Thetubes which are used with frequency measuring receiver A may beidentified by character AI to AI I. It will be seen'that the plates ofthis series of tubes are all connected. in parallel.

In a similar manner the plates of tubes BIto BII for use with receiver Bare connected in parallel, as are also the plates of tubes CI to OH forreceiver 0.

It may be seen from thefigure that the screens of the tubes AI to Allare connected to a series ofterminals. These terminals are cabled to asingle pole eleven position switch 2I3 at measuring receiver A. Thescreens of tubes BI to BII and CI to CI! arecabled to similar switcheslocated; on receivers; B vandv C,v respectively.

The rotary arm of each of these switches is connected to the screensupply voltage B+. Thus when the'arm of this switch connectslto terminalI screen voltage is applied to tube AI and when the arm is on terminalII screen voltage is. applied to tube Al I. Hence, only one of the tubesAl to, Al I is in an operating condition at any one time.

The common plate connections of tubesAI to All are connected through a30-50 kilocycle band-pass filter ZIS to converter tube D where thefrequencypassed by the tube which is in operating condition combineswith the fixed 760 kilocycle frequency from 2I2 to produce a frequencyin the range 790-810 kilocycles. This is filtered by a 790-810 kilocycleband-pass filter-.2 I 8 and is connected in measuring receiver A inplace of the 800 kilocycle oscillator shown in Figure 2 to grid83-oftube-80.

In a similar fashion,- the frequency passed by v the tubein series BI toBI I which is in operating condition is filtered, combined with thefixed :7 60

kilocyclesin converter tube E, is again filtered;

'This. isrepeateda third time for tubes CI to CI I and receiver 0.

Since these frequencies are obtained from the 1000 cycle frequencystandard, any-possibilityof V drift of the 800kilocycle oscillator iseliminated.

Oscilloscope 49 may also be eliminated since its only purpose was theaccurate setting of the frequencyof the 800 kilocycle oscillator andthis is now varied by selecting the desired frequency from the devicejust described.

In Figure ewehave shown a modification of the system of thepresentinvention wherebyfrequencies below kilocycles may. readily be.

measured. Y

In this figure there will be seen -at.the left severallines 300 comingfrom radio frequency amplifiers of various low frequency receivers (notshown). These receivers generally operate over the range from 16kilocycles to 80 kilocycles. The signals on any one of the lines 300 maybe selectively connected to the input of a frequency converter 39! bymeans of switch 302 where it is combined with 100 kilocycles from thefrequency standard. If we call the original frequency applied to theinput, f1, the new frequency will be 100 kilocycles-l-fi. The output ofthe frequency converter 3M passes through a high-pass filter 303 with acut-off frequency of 100 kilocycles or slightly higher to eliminateundesired frequencies in the output The output is then connected to ,7

the input of radio frequency amplifier 4 of Figure 1. In a frequencymeasuring system, a superhetercdyne receiver having a tunable oscillatortherein, a standard frequency source, means for receiving a signal waveto be measured, means for tuning said receiver to the 10 kilocycleharmonic of said standard frequency source which is nearest said signalwave, means for heterodyning said signal wave with the wave generated bysaid tunable oscillator to obtain a difference frequency, means forheterodyning the resultant diiference frequency with the beat frequencygenerated by' the mixing of the output waves of a pair of independentlycontrollable oscillators to obtain a first intermediate frequency wave,means for generating a fixed frequency wave of the same order ofmagnitude as said beat frequency, means for mixing said fixed frequencywave with said difference frequency to obtain a second intermediatefrequency, means for converting both of said intermediate frequencies toa lower frequency band. including the audible frequency range, means forcomparing said converted intermediate frequencies whereby saidindependently controllable oscillators may be so adjusted that saidconverted intermediate frequencies are equal to one another and meansfor measuring the difference between said beat frequency and said fixedfrequency.

2. In a frequency measuring system, a superheterodyne receiver having atunable oscillator therein, a standard frequency source, means forreceiving a signal wave to be measured, means for tuning said receiverto the 10 kilocycle harmonic of said standard frequency source which isnearest said signal wave, means for heterodyning said signal wave withthe wave generated by said tunable oscillator to obtain a differencefrequency, means for heterodyning the resultant difference frequencywith the output of a variable frequency oscillator to obtain a firstintermediate frequency wave, means for generating a fixed frequency waveof the same order of magnitude as said beat frequency, means for mixingsaid fixed frequency wave with said difference frequency to obtain asecond intermediate frequency, means for converting both of saidintermediate frequencies to a lower frequency band including the audiblefrequency range, means for comparing said converted intermediatefrequencies, variable frequency means for measuring the differencebetween said beat frequency and said fixed frequency, means for varyingthe frequency of said variable frequency oscillator to obtain identitybetween said converted intermediate frequencies and means for measuringthe magnitude of said variation.

3. In a frequency measuring system, means for receiving a signal wave tobe measured including a superheterodyne receiver having a tunableoscillator therein, said receiver being tunable to the harmonic of astandard frequency source which is nearest in frequency to that of saidsignal wave, means for heterodyning said signal wave with the wavgenerated by said oscillator to obtain a difference frequency,an'interpolation oscillator comprising a pair of independently variablewave generators and means formixing the waves generated thereby toobtain a beat frequency, means for comparing said beat frequency withsaid'difference frequency, one of said gen erators being variable overat least the greater part of the maximum possible comparison range andthe other over only a definite fraction of the range of said oneoscillator.

4. Ina frequency measuring system, means for receiving'a signal wave tobe measured including a superheterodyne receiver. having a tunableoscillator therein, said receiver being tunable to the 10 kilocycleharmonic of a standard frequency source which is nearest in frequency tothat of said signal-Wave, means for heterodyning said signal wave withthe wave generated by said oscillator to obtain a difference frequency,an interpolation oscillator comprising a pair of independently variablewave generators and means for mixing the waves generated thereby toobtain a beat frequency, means for comparing saidbeat frequency withsaid difference frequency, one of said generators being variable over atleast the greater part of themaximum possible comparison range and theother over only a definite, fraction of the range of said oneoscillator, the sum of the variations of saidgenerators being greaterthan the intervals between the harmonicsfrom said standard frequencysource. j

5. A frequency measuring system consisting of preselector circuits, afirst, converter, ayfirst branch and asecond branch, each branch-beingcoupled to said first converter-to receive a first intermediatefrequency output therefrom,

means in the first branch to indicate when the;

first intermediate I frequency is exactly equal to a certain locallygenerated constant frequency, means in the second branch including acalibrated variable frequency source for heterodyning the firstintermediate frequency to produce a second intermediate frequency, meansfor indicating when the second intermediate frequency is exactly equalto another locally generated con frequency can be accurately determinedby measuring with the calibrated variable frequency source thedifference between the known harmonic frequency and the unknownfrequency, the correct condition of adjustment of the first conyerterbeing indicated by the response to the known harmonic frequency in thefirst branch duringthe time required-to manipulate thecalibrated'variable frequency source in the second branch;

6. In a frequency measuring system, means for receiving a signal wave tobe measured including a superheterodyne receiver having a tunableoscillator therein, said receiver being tunable to the 10' kilocycleharmonic of a standard frequency source which is nearest in frequency tothat of said signalwave, means for heterodyning said signal wave withthe wave generated by said oscillator to obtain a difference frequency,an interpolation oscillator comprising a pair ofiindependently variablewave generators and means for mixing the waves generated thereby toobtain a beat frequency, means for comparing said beat frequency'withsaid difference frequency, one of said generators being variable in aplurality of definitely known steps over a 20 kilocycle band and theother continuously variable over a 2 kilocycle band.

'7. Ina frequency measuring system,.means for receiving a signal wave tobe measured including asuperheterodyne receiver having a tunableoscillator therein, said receiver beingtunable to the harmonic ofa'standard frequency source which is nearest in frequency to that ofsaid signallwave, means-for heterodyning said signal wave with the wavegenerated bysaid oscillator toobtain a difference frequency, aninterpolation oscillator comprising a pair of independently variableWave generators and means for mixing the waves generated thereby toobtain a beat-frequency, means for'comparing said beat frequency withsaid difference frequency, oneof said generators being variable over at"least the greater part of the maximum possible comparison range and theother over only'a definite fraction of therange of said one oscillator,and means for'comparingthe output of said interpolation oscillator witha standard frequency source whereby the frequency of 'said'interpolationoscillator in terms of multiples of said second standard frequency maybe determined.- 7

8: In'a frequency measuring system, means-for receiving a signal waveto, be measured including a superheterodyne-receiverhaving a tunableoscillatortherein, said receiver beingtunable to the 10 kilocycleharmonic of a standard frequency source whichis nearest in frequency tothat of saidsignal wave, means for'heterodyning said signal wave withthe wave generated by said oscillator to obtain a difference frequency,an interpolation oscillator comprising a pair of inde pendently variablewave generators and means for mixing the waves generatedthereby toobtain a beat frequency, means for comparing said beat frequency withsaid difference frequency, one of said generators being variable over atleast the greater part of the maximum possible comparison range and theother over only a definite fraction of the range of said one oscillator,and an oscilloscope for comparing the output of said interpolationoscillator with a'standard frequency source whereby the frequency ofsaidinterpolation oscillator in terms of multiples of 'said second stand:ard frequency may be determined.

9. In a frequency measuring system, means for receiving a signal wave tobe measured includinga superheterodyne receiver having a tunableoscillator therein, said receiver being tunable to the harmonic of astandard frequency source which is nearest in frequency to that ofsaidsignal'wave, means for heterodyning said signal wave with the waveenerated by said oscillator to obtain adifference frequency, anoscillator for generating another wave, means for mixing said other wavewith said difference frequency to produce a first intermediate frequencyand means for mixing said first intermediate frequency with the waveenergy of such frequency that for a predetermined difference frequency azero beat is obtained.

10. In a frequency measuring system, means for receiving a signal waveto be measuredincludinga superheterodyne receiver having a tunableoscillator therein, said receiver being tunable to the harmonic of astandard frequencysource which is nearest in frequency to that ofsaid'signal wave, means for heterodyning said signal wave with the wavegenerated by said oscillator'to obtain a difference frequency, aninterpolation oscillator comprising a pair of independently variablewave generators and means for mixing the wave generated thereby toobtain a beat frequency and means for comparing said beat frequency withsaid difference frequency, said last means'ineluding means forconverting the wave generated bythe mixing of said beat frequency anddifference frequency to a predetermined frequency for a knownrelationship between said beat frequency and difference frequency.

HAROLD O. PETERSON. JOHN B. ATWOOD.

